The present invention relates to an ultrasound signal processor used in an ultrasound imaging system such as an ultrasound diagnostic system or ultrasound non-destructive testing system, and more particularly to an ultrasound signal processor for processing received ultrasound signals on the basis of digital beamforming.
In the conventional ultrasound imaging system, an ultrasound pulse signal is transmitted into a testing body or patient and an echo signal from a location having a different specific acoustic impedance (or a reflector) is received to obtain a sectional image or the like. In this case, the reception is made in such a manner that different delay times are given to the respective channels of a group of parallel received signals on the basis of the assumption of a transmission time of sound wave between a specified position in the testing body and receiving elements (or electro-acoustic transducer elements) to make phase matching and an echo signal from a reflector is obtained by use of interference caused by coherent signal addition.
In general, if the numbers of parallel received signals is large, a larger receiving aperture can be set, thereby making it possible to improve the resolution for a focal point on receiving. For this purpose is used an synthetic aperture method in which an ultrasound transmitting and receiving process is performed plural times to make the addition while moving the position of a receiving aperture for the same focal point (see, for example, JP-A-52-112223). However, in the case where a reflector in a testing body is in activity or motion (particularly in the case of an ultrasound diagnostic system or the like), there is problem that the correct addition of coherent or in-phase signals is not possible since the position of the reflector in the testing body moves in a period of time when the ultrasound transmitting and receiving process is performed plural times.
As a broad-band transmission system in the research field of communication, there is widely known a technique in which channels are formed by dividing a signal into frequency bands and a plurality of signals are transmitted in a single transmission route in a band limited and compressed form by use of a modulation and demodulation technique based on the frequency shift of a band. Particularly in the transmission of sound or voice signals, there is known a single side band (SSB) transmission system.